1. Field of the Invention
This invention relates generally to structures produced by techniques of nanotechnology, and methods of producing such structures. More specifically, the invention relates to such structures and devices incorporating at least one element, essentially in one-dimensional form, which is of nanometer dimensions in its width or diameter, which is produced with the aid of a catalytic particle, and which is commonly termed a “nanowhisker.”
2. Background Art
Nanotechnology covers various fields, including that of nanoengineering, which may be regarded as the practice of engineering on the nanoscale. This may result in structures ranging in size from small devices of atomic dimensions, to much larger scale structures—for example, on the microscopic scale. Typically, nanostructures are devices having at least two dimensions less than about 1 μm (i.e., nanometer dimensions). Ordinarily, layered structures or stock materials having one or more layers with a thickness less than 1 μm are not considered to be nanostructures. Thus, the term nanostructures includes free-standing or isolated structures that have two dimensions less than about 1 μm, that have functions and utilities different from those of larger structures, and that are typically manufactured by methods different from conventional procedures for preparing somewhat larger, i.e., microscale, structures. Although the exact boundaries of the class of nanostructures are not defined by a particular numerical size limit, the term has come to signify such a class that is readily recognized by those skilled in the art. In many cases, an upper limit of the size of the at least two dimensions that characterize nanostructures is about 500 nm. In some technical contexts, the term “nanostructure” is construed to cover structures having at least two dimensions of about 100 nm or less. In a given context, the skilled practitioner will recognize the range of sizes intended. In this application, the term “nanostructure” is broadly intended to refer to an elongated structure having at least two transverse dimensions less than about 1 μm, as indicated above. In more preferred applications, such dimensions will be less than about 100 nm, more preferably less than about 50 nm, and even more preferably less than about 20 nm.
Nanostructures include one-dimensional nanoelements, essentially in one-dimensional form, that are of nanometer dimensions in their width or diameter, and that are commonly known as nanowhiskers, nanorods, nanowires, nanotubes, etc.
The basic process of whisker formation on substrates by the so-called VLS (vapor-liquid-solid) mechanism is well known. A particle of a catalytic material, usually gold, is placed on a substrate and heated in the presence of certain gases to form a melt. A pillar forms under the melt, and the melt rises up on top of the pillar. The result is a whisker of a desired material with the solidified particle melt positioned on top. See Wagner, Whisker Technology, Wiley, New York, 1970, and E. I Givargizov, Current Topics in Materials Science, Vol. 1, pages 79-145, North Holland Publishing Company, 1978. In early applications of this technique, the dimensions of such whiskers were in the micrometer range, but the technique has since also been applied for the formation of nanowhiskers. For example, International Patent Application Publication No. WO 01/84238 (the entirety of which is incorporated herein by reference) discloses in FIGS. 15 and 16 a method of forming nanowhiskers, wherein nanometer sized particles from an aerosol are deposited on a substrate and these particles are used as seeds to create filaments or nanowhiskers.
Although the growth of nanowhiskers catalyzed by the presence of a catalytic particle at the tip of the growing whisker has conventionally been referred to as the VLS (Vapor-Liquid-Solid process), it has come to be recognized that the catalytic particle may not have to be in the liquid state to function as an effective catalyst for whisker growth. At least some evidence suggests that material for forming the whisker can reach the particle-whisker interface and contribute to the growing whisker even if the catalytic particle is at a temperature below its melting point and presumably in the solid state. Under such conditions, the growth material, e.g., atoms that are added to the tip of the whisker as it grows, may be able to diffuse through a the body of a solid catalytic particle or may even diffuse along the surface of the solid catalytic particle to the growing tip of the whisker at the growing temperature. Persson et al., “Solid-phase diffusion mechanism for GaAs nanowires growth,” Nature Materials, Vol. 3, October 2004, pp 687-681, shows that, for semiconductor compound nanowhiskers there may occur solid-phase diffusion mechanism of a single component (Ga) of a compound (GaAs) through a catalytic particle. Evidently, the overall effect is the same, i.e., elongation of the whisker catalyzed by the catalytic particle, whatever the exact mechanism may be under particular circumstances of temperature, catalytic particle composition, intended composition of the whisker, or other conditions relevant to whisker growth. For purposes of this application, the term “VLS process,” or “VLS mechanism,” or equivalent terminology, is intended to include all such catalyzed procedures wherein nanowhisker growth is catalyzed by a particle, liquid or solid, in contact with the growing tip of the nanowhisker.
For the purposes of this specification the term “nanowhisker” is intended to mean a one-dimensional nanoelement with a width or diameter (or, generally, a cross-dimension) of nanometer size, the element preferably having been formed by the so-called VLS mechanism, as defined above. Nanowhiskers are also referred to in the art as “nanowires” or, in context, simply as “whiskers” or “wires.”
Freestanding nanowhiskers have drawn increasing attention for their potential use in applications in electronics and photonics.
As already shown in the early work of Wagner, referenced above, the preferential growth direction of such nanowhiskers is <111>. One drawback of <111> oriented nanowhiskers is the high density of stacking faults that commonly form perpendicular to the growth direction. These defects are expected to affect the physical properties of the nanowhiskers. Another drawback with this preferential growth direction is its non-compatibility with the (001) crystal face of the main surface of substrates commonly used in industrial applications. That is, the preferential growth direction is oblique rather than normal to the substrate main surface, the normal direction being <001>. For example, with III-V compounds, the commonly commercially available substrates have an (001) crystal face as the main surface. In contrast, nanowhiskers of III-V compounds preferentially grow in a <111>B direction from a (111)B crystal plane.
Hiruma et al., J. Appl. Phys., 77(2), 15 Jan. 1995, pages 447-462, reported the growth on InAs nanowhiskers on GaAs substrates having various surfaces including, in particular, the (001) surface. The InAs nanowires invariably grew in the <111> direction, resulting, for example, in pairs of wires ([1-11] and [-111]) tilted with an angle of 35° towards the (001) surface.
Other growth directions for nanowhiskers have been observed to occur sporadically during whisker growth. For instance, Wu et al, “Growth, branching, and kinking of molecular beam epitaxial <110> GaAs nanowires,” Applied Physics Letters, 20 Oct. 2003, Vol. 83, No. 16, pp 3368-3370, disclosed an <011> direction for GaAs nanowhiskers grown on GaAs (001) by molecular beam epitaxy (MBE).
Björk et al, “One-Dimensional Heterostructures In Semiconductor Nanowhiskers,” Applied Physics Letters, Vol. 80, No. 6, 11 Feb. 2002, pages 1058-1060, described an <001> segment of an InAs/InP heterostructured nanowhisker grown from a (111)B GaAs surface in chemical beam epitaxy (CBE), the segment having deviated from the initial <111>B growth direction of the nanowhisker. More particularly, it had been observed that whereas most nanowhiskers grew in the <111>B direction, there were sporadically formed nanowhiskers in the form of a “hockey-stick” that initially grew in the <111>B direction, but “kinked” to the <001> direction. The nanowhisker disclosed had a base region of InAs and grew in the <001> direction as a result of the compressive strain at the InP/InAs interface. Growth of a nanowhisker in the <001> direction dramatically reduces the formation of defects, such as stacking faults.
In International Patent Application Publication No. WO 2004/004927 (the entirety of which is incorporated herein by reference), there is disclosed in FIG. 24(b) a technique for controlling the growth direction of whiskers wherein, by applying strain to the whisker during formation, by change of growth conditions, the direction of growth of the whisker can be changed to the <100> direction from the usual <111> direction. Alternatively, a short bandgap segment of a wide bandgap material may be grown at the base of the nanowhisker.
Still further improvements in the control of the growth direction of nanowhiskers are desirable. For example, a method that would provide for an initial whisker growth direction normal to an incompatible substrate surface—that is, where the preferential growth direction of the whisker is oblique to the surface—would be highly desirable, as would structures produced by such method. Such a method would allow for the growth of whiskers that are normal to the surface over their entire length (or, more generally, at least the initial portion of their length) as opposed to the kinked nanowhiskers having an initial growth direction oblique to the surface as previously observed.